When a failure occurs in a communication network, communication may not be executed. In order to avoid this problem, the communication network is designed to be redundant. In the communication network, when a failure occurs in a certain path, traffic on the certain path is transmitted through another path.
In a redundant network, one or more loops are formed. In the network that has the one or more loops, when a broadcast frame or a multicast frame is transmitted, multiplexing called a broadcast storm occurs. As a result, communication resources are wastefully used. In the worst case, the network may be down.
As one of methods for improving the reliability while avoiding a loop on the network, the Spanning Tree Protocol (STP) is known. The Spanning Tree Protocol is a standard protocol defined in the IEEE802.1D recommendation. According to the Spanning Tree Protocol, when a loop is detected on a network, an adverse effect caused by the loop may be avoided by causing a port of a communication device to logically block traffic. According to the Spanning Tree Protocol, communication devices may detect a connection state of the network by transmitting and receiving bridge protocol data unit (BPDU) frames between the communication devices.
FIGS. 1A and 1B are diagrams describing the Spanning Tree Protocol. In examples of FIGS. 1A and 1B, communication devices 200A to 200C are connected to each other in a mesh form. IDs are provided to the communication devices 200A to 200C, respectively.
Each of the communication devices 200A to 200C periodically transmits a BPDU frame. The BPDU frame includes a “root ID”, a “path cost” and a “bridge ID”. The root ID identifies a communication device that operates as a root bridge. The path cost indicates a cost of communication to the root bridge. The bridge ID identifies a communication device that transmits the BPDU frame.
Each of the communication devices 200A to 200C may detect a connection state or topology of a network by analyzing BPDU frames received from the other adjacent communication devices. For example, the communication devices 200A to 200C may detect a loop on the network. The following description assumes that the communication device 200A is the root bridge. The root bridge is determined on the basis of the IDs provided to the communication devices 200A to 200C, for example.
In FIG. 1A, the communication device 200A transmits a BPDU frame A through ports P1 and P2. In this case, a “root ID A” and a “bridge ID A” are set in the BPDU frame A. The communication device 200B receives the BPDU frame A through a port P1. The communication device 200C receives the BPDU frame A through a port P1.
The communication device 200B transmits a BPDU frame B through a port P2. In this case, the “root ID A” and a “bridge ID B” are set in the BPDU frame B. The communication device 200C receives the BPDU frame B through a port P2.
In this case, the communication device 200C receives the BPDU frames A and B with the set “root ID A” through the plurality of ports (P1 and P2). Thus, the communication device 200C detects that a plurality of paths physically exist between the root bridge (or the communication device 200A) and the communication device 200C. In other words, the communication device 200C detects a loop.
The communication device 200C sets one of the ports P1 and P2, as a blocking port. In this case, the communication device 200C determines one of the ports P1 and P2 that is to be set as the blocking port, on the basis of the costs of the paths that extend to the root bridge from the ports P1 and P2. In this example, the port P2 is set as the blocking port. In this case, the communication device 200C logically blocks a frame received through the port P2. In addition, the communication device 200C does not transmit a frame through the port P2. As a result, the loop on the network is released. A BPDU frame, however, is not blocked by the blocking port.
As a related technique, a method for maintaining a network operation without a reconfiguration of a network upon installation of an additional device using the Spanning Tree Protocol or upon the occurrence of a failure has been proposed (for example, Japanese Laid-open Patent Publication No. 2002-330152).
IEEE Std 802.1D, IEEE Standard for Local and Metropolitan Area Network: Media Access Control (MAC) Bridge, 9 Jun. 2004 is another example of related art.